1. Field of the Invention
The present invention relates to a drive transmission device that connects a drive shaft to a driven shaft in order to transmit a drive force from a drive source, and also relates to an image forming apparatus, such as a printer and a copier, equipped with the drive transmission device.
2. Description of the Related Art
Generally, a typical mechanical apparatus has a drive source, such as a motor and an engine, which generates a drive force. In such a device, the drive force of the drive source is transmitted from a drive shaft to a driven member through a drive transmission mechanism, such as a gear, a timing belt, and a clutch. Specifically, in a case where the rotating speed of the driven member is to be made adjustable or speed control is to be performed, which includes speed increasing and decreasing, it is necessary to set a reduction gear ratio by means of a complicated gear arrangement or to control the rotating speed of the drive source.
However, a complicated gear arrangement can lower the drive transmission efficiency and generate vibration resulting from gear mesh frequency. Furthermore, the rotating-speed control of the drive source depends heavily on the torque characteristics of the drive source itself, which implies that there is a limitation on the effective rotating-speed range. Especially in a case where the drive source is a motor, there is a limitation on the responsiveness, such as the activation time. Although there is known a technique that employs a clutch to switch between drive modes, this technique can be problematic in that it must be taken into consideration a transmission shock occurring at the time of on/off switching of the drive modes and abrasion occurring in the transmission portion.
As a device that achieves higher drive transmissibility without the various limitations created in above-described devices that employ mechanical drive transmission mechanisms, a drive transmission device that employs an electro-rheological (ER) fluid is known. An electro-rheological fluid is a kind of fluid whose viscosity changes instantaneously and reversibly in accordance with an electric field intensity applied thereto. Generally, the viscosity of an electro-rheological fluid increases with increasing electric field intensity.
Japanese Patent Laid-Open No. 6-107026 and Japanese Patent Laid-Open No. 2005-195171 disclose examples of a clutch which includes a drive member and a driven member that are engaged to each other but have a gap therebetween that is filled with an electro-rheological fluid. In these examples, the viscosity of the electro-rheological fluid, namely, the slippage between the drive member and the driven member, can be controlled by changing the electric field intensity applied to the electro-rheological fluid. Similarly, Japanese Patent Laid-Open No. 7-174168 discloses an example of a coupling that utilizes the viscosity changeability of the electro-rheological fluid to control transmission distribution of a drive force.
These drive transmission devices employing an electro-rheological fluid are characterized in that a drive shaft and a driven shaft are linked to each other in a slipping fashion. In other words, a state of a half clutch can be controlled very smoothly and stably. In contrast, in a mechanical clutch mechanism, vibration and abrasion occur since clutch plates slide against each other in a half clutch state, and it is thus difficult to stably maintain the slippage between the clutch plates. The use of an electro-rheological fluid however allows for stable maintainability of the slippage in accordance with the applied electric field intensity.
Consequently, the use of an electro-rheological fluid, whose viscosity changes continuously with good responsiveness, allows for transmissibility of a drive force by a required amount, whereby vibration and abrasion occurring from connected driving can be reduced. Moreover, the use of an electro-rheological fluid also allows for driving methods such as gearless driving and motor control-free driving by half clutch control.
However, simply controlling the slippage between the drive shaft and the driven shaft is not sufficient if the speed of the driven member is to be changed within a short period of time or if the rotating speed of the driven member is to be controlled precisely to a constant rate. This is because, although increasing the rotating speed can be readily achieved by increasing the viscosity of the electro-rheological fluid, decreasing the rotating speed requires a separate brake mechanism for arbitrarily controlling the rotating speed to a lower rate. This means that even when the electric field is cut off, namely, when the clutch becomes completely in a disengaged state, the driven member will continue to rotate due to inertia and will not slow down right away.
If a precise operation, such as changing the rotating speed of the driven member within, for example, several tens of milliseconds, is not necessary, a sufficient effect can be achieved without a brake mechanism. However, in the case of a mechanical apparatus such as an image forming apparatus in which the rotation accuracy and timing of an image bearing member or a sheet conveying roller are extremely important and require precision, the control will not be sufficient without a brake mechanism in terms of time.
Although it is possible to provide the apparatus with an additional brake mechanism that is separate from the drive transmission device so that the driven member can be decelerated or stopped within a short period of time, the additional brake mechanism will result in an increase in the size of the apparatus as well as the costs.